1. Field of the Invention:
This invention relates generally to the enhancement of pel-generated (raster) displays wherein the pel resolution is increased and pels at the higher resolutions are added or deleted to take advantage of the increased resolution. It relates particularly to enhancements to displays of combined text and graphics, the latter in the form of half-tone images wherein the text is enhanced without degrading the half-tone images.
2. Description of Related Art:
Raster displays, which include CRTs and all-points-addressable (APA) printers, present information using pels, sometimes referred to as dots or pixels. Pels are small sections of an image, derived from the words "picture elements." Images and text are formed by exhibiting pels, e.g., as lighted spots on a CRT screen or as ink spots on a printer, at a certain resolution. A common resolution is 240 pels per inch. That is, each square is divided into 240-by-240 pels or a density of 57,600 pels per square inch.
Usually text characters and prepared half-tone images are positioned on a page (or bit) map at the desired location on the page in a storage device. A bit corresponds to a pel in black or white. For colors or gray scales, several bits per pel are required to specify the pel's attributes. In the following description, a bit corresponds to a pel. If the pel is to be displayed as a lighted spot or ink dot, then it has a value of logical 1 or is referred to as set and the pel is referred to as a black pel. If the pel is not to be displayed, it has a value of logical 0 or is referred to as reset, and the pel is referred to as a white pel.
When the resolution of the displayed information is to be increased, the preset characters and images can be redetermined for use at the higher resolution. Considerable effort, however, may be required to redefine the characters and images at the higher resolution. Consequently, it may be desirable to use the preset characters at the higher resolution and images designed for the lower resolution. Usually, to accomplish such a substitution, the higher resolution is made an integral multiple of the lower resolution. For example, if 240-pel characters and images are available, the higher resolution might be 480 pel per inch resolution, i.e., 230,400 pels per square inch.
In addition to the economy of not having to redefine text characters, the amount of storage for a page can be reduced. For example, an 8.5.times.11-inch page requires 5,385,600 bits at 240-pel resolution but 21,542,400 bits at 480-pel resolution.
Each pel at the 240-pel resolution can be replaced by four pels--a two-by-two block--of the same value at the 480-pel resolution. Such replacement, however, does not take advantage of the higher resolution. The text and images would have the same granularity. Stair-stepping, the uneven edges of a sloped line, would still be as noticeable. It is, therefore, desirable to enhance the image by replacing lower resolution pels with a pattern of higher resolution pels that reduces the stair-stepping. For example, a black pel at the lower resolution may be replaced with a pattern of three black and a white pel at the higher resolution, the position of the white pel depending on the slope of the line and which edge it depicts.
U.S. Pat. No. 4,437,122 (Walsh et al.) shows a method for enhancing slope lines or curve lines at a higher resolution three times the lower resolution. Each pel at the lower resolution is replaced by a three-by-three pattern of pels at the higher resolution, some of the pels being black, some of the pels being white, depending on certain characteristics of the line with respect to slope or curvature. This scheme has the disadvantage that, when used on information representing mixed text and halftone images, tends to degrade the halftone image while enhancing the text portion.
Gray tones in printing cannot be reproduced by varying ink density. In offset printing, graphic arts photography creates a plate negative using a halftone screen, converting continuous tone images into small, varying size dots. The dots, when printed, create an optical illusion of gray tones because, even though well defined, the individual dots are not perceived but are blended by the visual process. Larger dots produce darker gray tones than smaller dots.
In raster printing, e.g., all points addressable printers, the individual dots (pels) are the same size. The apparent dot size is varied by combining pels. Several algorithms exist to control the manner of combining pels to increase dot size, thereby creating darker gray tones.
As a result of the algorithmic process, the pels in a halftone image are very structured and nonlocalized whereas the pel patterns of text characters are less structured and more localized. The invention described and claimed herein use these differing characteristics of text and halftone images to enhance text at the higher resolution while not degrading intermixed halftone images.